Electroless copper plating bath

ABSTRACT

High stability electroless copper plating baths containing as a stabilizer a small amount of o-phenanthroline and of iodide ions.

United States Patent Assignee Frank E. Stone Trumbull, Conn. 789,402

Jan. 6, 1969 Oct. 26, 1971 Enthone, Incorporated New Haven, Conn.

Inventor Appl. No. Filed Patented ELECTROLESS COPPER PLATING BATH 23Claims, No Drawings s, (I 106/1, 117/47 A, 117/50, 117/160 R int. 844d1/092,

Primary Examiner-Alfred L. Leavitt Assistant Examiner-Janyce A. BellAttorneys-Elwood J. Schaffer and Roger J. Drew ABSTRACT: l-lighstability electroless copper plating baths containing as a stabilizer asmall amount of o-phenanthroline and of iodide ions.

ELECTROLESS COPPER PLATING BATH BACKGROUND OF THE INVENTION 1. Field ofthe Invention This invention relates to electroless copper plating, andmore particularly to new, high stability chemical reduction copperplating solutions. Additionally this invention relates to newconcentrate compositions and additive compositions especially wellsuited for use in forming the high stability electroless copper platingsolutions herein and for addition thereto as stabilizer, and to a methodfor the electroless plating of copper involving the use of the highstability electroless copper. 2. Description of the Prior ArtElectroless copper plating is of considerable importance commercially. Aprincipal application is in the electronics industry, for instance inthe production of printed circuits. In the latter use, electrolesscopper is employed to render plastic surfaces electrically conductive,especially in the manufacture of plated-through hole printed circuitboards. Other applications in the electronics industry take advantage ofthe high electrical conductivity of thin deposits of electroless copper,as in the coating of epoxy surfaces to produce additive type printedcircuits and the plating of magnetic tape.

Commercial formulations of electroless copper plating compositions areusually supplied as two solutions or powders with the copper salt in onesolution or powder and the reducing agent, frequently formaldehyde, inthe other. The two solutions or powders and water are mixed togetherjust prior to use, whereby the reduction of the copper ions commencesshortly thereafter to plate out metallic copper. In certain commercialformulations, in the case of the solutions, both the copper salt andreducing agent may be in one of the solutions with the pH of thesolution maintained low enough and on the acid side to prevent the redoxreaction, and the other solution, which is an alkaline solution, willcontain the alkali and complexing agent for cupric ions.

The prior art electroless copper plating solutions are characterized bybeing very unstable compositions and undergoing a spontaneousdecomposition after only a short period of life, which is a matter oftypically a few hours. The copper precipitates out of the solution andmetallic copper particles can usually be observed at the bottom of thetank or container holding the solution. The decomposed solutions areusually not regenerable or recoverable, and must be discarded.

2, 9-Dimethylphenantroline and 4, 7-dipheynyl-2, 9- dimethyl-l,IO-phenanthroline have been reported heretofore as stabilizers forelectroless copper plating solutions. The mechanism of such compoundshas been to form a chelated compound selectively with cuprous ion in thesolution thereby stabilizing the solution.

SUMMARY OF THE INVENTION In accordance with the present invention, Ihave found that alkaline electroless copper plating solutions areprovided of considerably greater stability than the prior electrolesscopper plating solutions'previously referred to herein by incorporatinginto or adding to the plating solution as stabilizera" small, effectiveamount of o-phenanthroline and of iodide ions. The electroless copperplating solutions or baths of this invention were found to be ofconsiderably greater stability than alkaline electroless copper platingsolutions stabilized with iodide ion alone, o-phenanthroline alone, and2, 9-dimethyl phenanthroline alone, and this considerably greaterstability provided by the synergistic combination of the ophenanthrolineand iodide ions was unexpected and surpris- Ihe high stability.electroless copper plating solution or bath of this invention, in itsbroader aspects, comprises an alkaline aqueous solution containingcopper ions, a complexing agent for cupric ion, a reducing agent for theionic copper, and as the stabilizer, a small effective amount ofo-phenanthroline and of iodide ions.

The small amount of o-phenanthroline and of iodide ions is an amountsufficient to stabilize the bath against its premature decomposition.However, the amount of o-phenanthroline and of iodide ions utilized inthe electroless copper plating baths herein should be less than thatamount of such ingredients which will prevent copper from beingdeposited from the bath.

This invention also comprehends concentrate compositions especially wellsuited for use in forming the high stability electroless copper platingsolutions herein, and in their broader aspects each comprising a mixtureof a water-soluble copper salt, a reducing agent for ionic copper, andsmall amounts of o-phenanthroline and of a source of iodide ions,usually a water-soluble iodide, for example an alkali metal iodide, e.g.sodium or potassium iodide. The small amounts of ophenanthroline and ofthe source of iodide ions in the concentrate composition are amounts ofsuch materials insufficient to prevent copper from depositing from theultimate electroless copper plating bath, and which are sufficienteither with or without separately added o phenanthroline and iodide ionsource to stabilize the electroless copper plating bath.

In still another embodiment of this invention, additive compositions areprovided for addition as stabilizer to alkaline electroless copperplating solutions or baths. The additive composition herein, in itsbroader aspects, comprises a mixture of o-phenanthroline and a source ofiodide ions.

Other ingredients can also be present in the additives, if desired, suchas, for example, an inert diluent, e.g. an inert aqueous liquid solvent,usually water, the alkali metal hydroxide and/or the alkali metalcarbonate.

The source of the copper ions in the plating solutions herein is usuallya water-soluble copper salt, e.g. copper sulfate, copper nitrate orcopper chloride.

The complexing agent for cupric ion is usually Rochelle salt, althoughother suitable complexers for cupric ion may be utilized instead of theRochelle salts, if desired. Exemplary of such other complexing agentsare ethylenediaminetetraacetic acid and alkali metal and alkaline earthmetal salts thereof, nitrilotriacetic acid and alkali metal and alkalineearth metal salts thereof, ethylenediamine and certain derivativesthereof, and amino acid complexing agents.

Although it is not known with certainty, it is believed the iodide ionand o-phenanthroline stabilize the electroless copper plating baths byforming a co-complex or addition compound with cuprous ion. Theco-complex or addition compound is possibly either (I) and wherein a anda denote the l and 10 positions of ophenanthroline in all of suchformulas l (2) and (3).

DESCRIPTION OF THE PREFERRED EMBODIMENTS The o-phenanthroline ispreferably present in the electroless copper plating baths of thisinvention in an amount in the range of 1X10 to l l0 gram per liter. Theiodide ion is preferably present in the electroless copper plating bathsin an amount equivalent to 1X10 to 1X10 gram per liter of sodium iodideor potassium iodide.

Formaldehyde is the preferred reducing agent herein and is convenientlyadded as an aqueous solution, such as formalin. Compounds or materialscapable of providing formaldehyde The additive compositions llerein canbe a liquid or solid, or of a consistency between a liquid and solid,e.g. a semiliquid or semisolid. The additive compositions can containthe mixture of o-phenanthroiine and the iodide ions, or source of iodidehe considerable improvement in stability of the bath provided by thecombination of iodide ions and o-phenanthroline as additive over theo-phenanthroline alone in the bath, the 2, 9-dimethyl-o-phenanthrolineused alone in the bath, and no ions, for example an alkali metal iodide,e.g. Nal or Kl, within 5 such additives in the bath, is shown by thetest results of table quite broad proportion ranges so long as theamounts of such 2.

materials are insufficient to prevent copper from depositing In table 2and also in table 3 hereinafter set forth, the comfrom the ultimateelectroless copper plating bath. The solid position of each Enplate Cu402A is the same as that of the additive compositions will typicallycontain amounts of o- Enplate Cu-402A of table 1 previously set forthherein, and phenanthroline and the alkali metal iodide within the proporth C mPO iti H 0f Enplate Cu-402B is the same as that of tion ranges of,by weight, from about l.0-99.0 percent of the each Enplate Gil-402BOftable 1 hereinh mh ]i d f about 99,041) percent f h 1. Additionaltesting was carried out to compare the stability kali metal iodide. Theliquid additive compositions containing of Prior chemical reduction ppPlating baths containing the iodide ions and o-phenanthroline, which areusually aque- I 5 y iodide ions as the additive, iodide i n plus 0- oussolutions, will typically contain from about 0.07-7 gramsPheflanthfoline as the additive, and n0 additi The test per liter of theiodide ions and from about .07-7 grams per results are selfol'th inTable 3 which l ws:

liter of the o-phenanthroline.

Tests were carried out to compare the stability of chemical TABLE 3reduction copper plating baths containing only iodide ions as Test Runadditive, only o-phenanthroline as additive, iodide ions plus 0- 32phenanthroline as additive, and no additive. The results are set forthin table 1 which follows: gggg ggg fj g gl-jf; j 30 300 300 300 The testresults of table 1 show the conslderable improvement in stability of thebath provided by the combination of the iodide ions and o-phenanthrolineas additive over either of these materials used alone in the bath or inthe absence of any such additives in the bath.

In table 1, the composition of each Enplate Cu-402A was:

The conslderable improvement ln stablllty of the bath proby weight videdby the combination of iodide ions and o-phenanthroline as the additiveover the iodide ions alone as additive and no additive is also shown bythe test results of table 3. g k jsgg gi While the novel features of theinvention have been dis- Fgrmaldehyde 37% closed herein and arespecified in the annexed claims, it will up 54.57 be understood thatvarious omissions, substitutions and changes may be made by thoseskilled in the art without departing from the spirit of the invention.and the composition of each Enplate Cu-402B was: 1 claim! 1. Anelectroless copper alkaline plating solution comprisfiwy Wish, ing anaqueous alkaline solution containing copper ions, a complexing agent forcupric ion, a reducing agent for the H20 780 copper ions, and as astabilizer a small amount, sufiicient to N30 stabilize the solutionagainst spontaneous decomposition, of o- N co, 1.44 phenanthroline andof iodide ions, the amount of said 0- phenanthroline and of said iodideions being insufficient to prevent copper from depositing from saidsolution.

2. The plating solution of claim 1 wherein the reducing Enplate is atrademark of Enthone, Incorporated. D.M. agent is formaldehyde. H O" intable 1 and in tables 2 and 3 hereinafter set forth 5 3. The platingsolution of claim 1 further characterized by means deionized water,which is water that has been purified containing an alkaline compound inamount sufficient to proby removal of foreign ions therefrom. vide thesolution with a pH in the range of 10 to 14.

Additional tests were conducted to compare the stability of 4. Theplating solution of claim 3 wherein the iodide ion is prior chemicalreduction copper plating baths containing present in amount equivalentto about 0.000l-0.01 g./l. of iodide ions plus o-phenanthroline asadditive, only 0- sodium iodide or potassium iodide and theo-phenanthroline phenanthroline as additive, only 2, Q-dimethyl-oin amofaboul 00001-00 g-llphenanthroline as additive, and no additive. Theresults are set Th p ng l i n f la m 3 h r n h lk in mforth in table 2which follows: pound is an alkali metal hydroxide.

TABLE 2 Test Run izfigifii e ll oii l fie s s ii .if? 1 300 300 300 300Enplate oil-402A (1008854)..- 300 300 Enplate Cu-402B (1K09750)... 300

Enplate Cu-402B (1L15766). 300 300 Enplate Cu-402B (11515852)..-

5 11.11. in CHaOH. 2 High gloss- Low gloss.

4 Static.

' 5 Static; liner broke after 4 days.

5 Aerated; during and after plating (L hr.) only. 7 Decomposition after16 days=%. Decomposition alter 16 days=16%.

6 The plating solution of claim further characterized by containing analkali metal carbonate in addition to the alkali metal hydroxide, andwherein the reducing agent is formaldehyde.

7. The plating solution of claim 5 wherein the alkali metal hydroxide issodium hydroxide and the alkali metal carbonate is sodium carbonate.

8. The plating solution of claim 7 wherein the cupric ion complexingagent is Rochelle salt.

9. A composition for use in electroless copper plating baths comprisinga mixture of o-phenanthroline and a source of iodide ions.

10. A liquid additive composition for electroless copper plating bathscomprising a mixture of o-phenanthroline, iodide ions and an aqueousliquid solvent.

11. A concentrate composition especially well adapted for use in forminghigh stability electroless copper plating solutions comprising a mixtureof a water-soluble copper salt, a

'reducing agent for ionic copper, o-phenanthroline, and a source ofiodide ions, the amount of o-phenanthroline and of the iodide ion sourcebeing a small amount of each sufficient to stabilize the ultimateelectroless copper plating solution against spontaneous decompositionbut insufiicient to prevent copper from depositing from said platingsolution.

12. An aqueous acid concentrate solution especially well adapted for usein forming high stability electroless copper plating solutionscomprising an aqueous acid solution of a water-soluble copper salt, areducing agent for ionic copper, o-phenanthroline, and a water-solublesource of iodide ions, the pH of said concentrate solution beingsufficiently below pH 7. to prevent any substantial precipitation ofcopper therefrom as a cupric hydroxide-containing precipitate and asmetallic copper by a redox reaction, the amount of ophenanthroline andof the iodide ion source being a small amount of each sufficient tostabilize the ultimate electroless copper plating solution againstspontaneous decomposition but insufficient to prevent copper fromdepositing from said plating solution.

13. The concentrate of claim 12 wherein the source of iodide ions is amaterial selected from the group consisting of sodium iodide andpotassium iodide.

14. The concentrate solution of claim 12 wherein the reducing agent forionic copper is formaldehyde, and also containing an acid material inamount sufficient to maintain the pH of the concentrate solutionsufficiently below pl'l7 to prevent any substantial precipitation ofcopper therefrom as a cupric hydroxide-containing precipitate and asmetallic copper by a redox reaction.

15. An aqueous acid concentrate solution especially well adapted for usein forming high stability electroless copper plating solutionscontaining the following constituents in proportions within thepercentage range specified:

In by Weight from about 0.02-0.04 from about 5-9 from about 35-45H,SO,(Analytic reagent grade CuSO.-SH,O HCHO (37% concentration)o-Phenanthroline from about 3 lO"-3Xl0 Nal or KI from about 3Xl0-3Xl0'H,0 balance,

Soluble Copper salt 20-26 g./I. Rochelle salt 4055 g./l.

Alkali metal hydroxide l0l 8 gJl. Formaldehyde (37% concentration -150ml./l. o-Phenanthrolme lxl0"-|Xl0" g./l. Sodium iodide lXl0-l l0" g./l.Alkali metal carbonate 2-l0 g./l.

the solution having a pH in the range of l0 to l4.

17. The plating solution of claim 16 wherein the copper salt is coppersulfate.

18. An electroless copper aqueous alkaline plating solution containingthe following constituents in amounts within the proportion rangesspecified:

Soluble Copper Salt 20-26 g-/l. Rochelle salt 40-55 g./l. Alkali metalhydroxide lO-IS g./l.

Formaldehyde (37% concentra tion) l00-l50 ml./l. o-PhenanthrolinePotassium iodide Alkali metal carbonate the solution having a pH in therange of 10 to l4.

19. An additive composition for electroless copper plating bathscomprising a mixture of o-phenanthroline, a source of iodide ions, andan inert diluent.

20. The composition of claim 19 wherein the diluent is at least onematerial selected from the group consisting of an alkali metal hydroxideand an alkali metal carbonate.

2]. The composition of claim 19 wherein the iodide ion source is analkali metal iodide.

22. The composition of claim 21 wherein the alkali metal iodide issodium iodide.

23. The composition of claim 21 wherein the alkali metal iodide ispotassium iodide.

mg? UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 36] 5 736 Dated Q t h 26 192] Inventor(s) Frank E. Stone It is certifiedthat error appears in the aboveidentified' patent and that said LettersPatent are hereby corrected as shown below:

C Column 1, line 48, "2,9-Dimethylphenantroline" should read --2,9-Dimethylphenanthroline--; line 48, "4,7-dipheyny1-2,9" should read--4,7-diphenyl-2,9- Column 2, line 69,

"lxle to lx10 should read lxl0- to lxl0' line 71,

"lxlo to 1xl0 should read --lxl0" to 1110- Column 3, line 29, "lxlo lxlOshould read --lxlolxlO line 30,

"lxlo lxl0 should read --lxlolxlO' line 72, "3'xl0 3xl0 should read--3Xl0- 3110- line 73 "3X10 1/3 3xl0 should read --3xl0' 3xl0' line 74,

"H2594" should read H S0 Column 4, line 13, under Test Run 16 of Table1, 0.0005" should read o.0005--. Column 5 line 33 'CuSO4- 5H20" shouldread --CuS04'5H20--. Column 7, line 5, "5" should read ---6---. Column8, line 7, "3x10 3x10 should read --3xl0' 3x10' line 9, "3x10 3xl0should read --3x10' 3xl0' line 23, "lxl0 lxlO should read --lulo' lxlo'line 24, "1x10 lx10 should read --lxl0' lxl0- line 37, "1x10 lxl0 shouldread --1xl0 lxlO line 38, "lxl0 lxl0 should read --lxl0- 1x1O' Signedand sealed this 18th day of April 1972.

ISEAL) Lttest "DNAHD MJLETCHFR, JR. ROBI BT GOT'ISCHALK J ['t-testingOfficer Commissioner of Patents

2. The plating solution of claim 1 wherein the reducing agent isformaldehyde.
 3. The plating solution of claim 1 further characterizedby containing an alkaline compound in amount sufficient to provide thesolution with a pH in the range of 10 to
 14. 4. The plating solution ofclaim 3 wherein the iodide ion is present in amount equivalent to about0.0001-0.01 g./l. of sodium iodide or potassium iodide and theo-phenanthroline in amount of about 0.0001-0.01 g./l.
 5. The platingsolution of claim 3 wherein the alkaline compound is an alkali metalhydroxide.
 6. The plating solution of claim 5 further characterized bycontaining an alkali metal carbonate in addition to the alkali metalhydroxide, and wherein the reducing agent is formaldehyde.
 7. Theplating solution of claim 5 wherein the alkali metal hydroxide is sodiumhydroxide and the alkali metal carbonate is sodium carbonate.
 8. Theplating solution of claim 7 wherein the cupric ion complexing agent isRochelle salt.
 9. A composition for use in electroless copper platingbaths comprising a mixture of o-phenanthroline and a source of iodideions.
 10. A liquid additive composition for electroless copper platingbaths comprising a mixture of o-phenanthroline, iodide ions and anaqueous liquid solvent.
 11. A concentrate composition especially welladapted for use in forming high stability electroless copper platingsolutions comprising a mixture of a water-soluble copper salt, areducing agent for ionic copper, o-phenanthroline, and a source ofiodide ions, the amount of o-phenanthroline and of the iodide ion sourcebeing a small amount of each sufficient to stabilize the ultimateelectroless copper plating solution against spontaneous decompositionbut insufficient to prevent copper from depositing from said platingsolution.
 12. An aqueous acid concentrate solution especially welladapted for use in forming high stability electroless copper platingsolutions comprising an aqueous acid solution of a water-soluble coppersalt, a reducing agent for ionic copper, o-phenanthroline, and awater-soluble source of iodide ions, the pH of said concentrate solutionbeing sufficiently below pH 7 to prevent any substantial precipitationof copper therefrom as a cupric hydroxide-containing precipitate and asmetallic copper by a redox reaction, the amount of o-phenanthroline andof the iodide ion source being a small amount of each sufficient tostabilize the ultimate electroless copper plating solution againstspontaneous decomposition but insufficient to prevent copper fromdepositing from said plating solution.
 13. The concentrate of claim 12wherein the source of iodide ions is a material selected from the groupconsisting of sodium iodide and potassium iodide.
 14. The concentratesolution of claim 12 wherein the reducing agent for ionic copper isformaldehyde, and also conTaining an acid material in amount sufficientto maintain the pH of the concentrate solution sufficiently below pH7 toprevent any substantial precipitation of copper therefrom as a cuprichydroxide-containing precipitate and as metallic copper by a redoxreaction.
 15. An aqueous acid concentrate solution especially welladapted for use in forming high stability electroless copper platingsolutions containing the following constituents in proportions withinthe percentage range specified: % by Weight H2SO4(Analytic reagent gradefrom about 0.02-0.04 CuSO4.5H2O from about 5-9 HCHO (37% concentration)from about 35-45o-Phenanthroline from about 3 X 10 5-3 X 10 3NaI or KIfrom about 3 X 10 5-3 X 10 3H2O balance, the pH of said concentratesolution being sufficiently below pH 7 to prevent any substantialprecipitation of copper therefrom as a cupric hydroxide-containingprecipitate and as metallic copper by a redox reaction.
 16. Anelectroless copper aqueous alkaline plating solution containing thefollowing constituents in amounts within the proportion rangesspecified: Soluble Copper salt 20-26 g./l. Rochelle salt 40-55 g./l.Alkali metal hydroxide 10-18 g./l. Formaldehyde (37% concentra-tion100-150 ml./l. o-Phenanthroline 1 X 10 4-1 X 10 2 g./l. Sodium iodide 1X 10 4-1 X 10 2 g./l. Alkali metal carbonate 2-10 g./l. the solutionhaving a pH in the range of 10 to
 14. 17. The plating solution of claim16 wherein the copper salt is copper sulfate.
 18. An electroless copperaqueous alkaline plating solution containing the following constituentsin amounts within the proportion ranges specified: Soluble Copper Salt20-26 g./l. Rochelle salt 40-55 g./l. Alkali metal hydroxide 10-18 g./l.Formaldehyde (37% concentra-tion) 100-150 ml./l. o-Phenanthroline 1 X 104-1 X 10 2 g./l. Potassium iodide 1 X 10 4-1 X 10 2 g./l. Alkali metalcarbonate 2-10 g./l. the solution having a pH in the range of 10 to 14.19. An additive composition for electroless copper plating bathscomprising a mixture of o-phenanthroline, a source of iodide ions, andan inert diluent.
 20. The composition of claim 19 wherein the diluent isat least one material selected from the group consisting of an alkalimetal hydroxide and an alkali metal carbonate.
 21. The composition ofclaim 19 wherein the iodide ion source is an alkali metal iodide. 22.The composition of claim 21 wherein the alkali metal iodide is sodiumiodide.
 23. The composition of claim 21 wherein the alkali metal iodideis potassium iodide.